1. Field the Invention
The present invention relates to a judging apparatus that judges an amount of charge in a battery. More specifically, the present invention relates to a judging apparatus which detects a charged state of a battery of an electric automobile by formulating a characteristic (that is, a relation among an electric energy, a residual capacity and a current) of a battery to estimate a residual capacity of the battery so that it is possible to estimate an available time for which the automobile will be able to run off of the battery, and it is further possible to notify a driver of the automobile of the need to recharge the battery.
2. Description of the Related Art
A residual operating time of a battery is not determined merely by a difference between a rated capacity and a quantity of electric charge or discharge to or from the battery, but is largely affected by the temperature of the battery, the magnitude of a discharged current, and a downtime between charging or discharging of the battery. For these reasons, it has been difficult to accurately estimate a residual operating time of a battery. To solve this problem, Japanese Patent Application Laid-Open Publication No. 2-187679 teaches a technique to store a relation among a temperature, a charged or discharged current, a voltage (or an internal resistance of a battery) and a residual operating time of the battery in a ROM.
A charge state of a battery can be estimated by subtracting an integrated value of a discharged current, that is, a quantity of electric charge, from a full charge capacity. This technique estimates a relatively accurate residual capacity of a battery when the discharge is at a constant current. This technique, however, cannot provide an accurate residual capacity of a battery if the consumed current of the battery fluctuates greatly according to a running state of a vehicle, for example, an electric automobile.
There is disclosed in Japanese Patent Application Laid-Open Publication No. 4-368401 an apparatus that calculates an operating rate of a battery from a temperature of the battery and a discharged current to estimate a current residual capacity of the battery from a previously detected residual capacity, an operating rate from the previously detected residual capacity and a current discharged between the previous detection time and the current detection time.
The technique disclosed in Japanese Patent Application Laid-Open Publication No. 2-187679 has a disadvantage in that the volume of data to be stored in a ROM is enormous because a relation among a charge or discharge current, a voltage and a temperature of a battery for every pattern of various running conditions of a vehicle need to be stored.
On the other hand, the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 4-368401 has a disadvantage in that it is impossible to accurately estimate a threshold at which the battery requires a recharge because actual batteries used to drive vehicles have variations in characteristics, or the previously detected residual capacity may change due to a number of factors, such as deterioration during use of the battery.